The subject invention pertains to thermoset interpolymers, to a process for their preparation, and to products fabricated from such thermoset interpolymers.
In one preferred embodiment, the present invention further pertains to foams prepared from such thermoset interpolymers and to methods for the preparation of cross-linked .alpha.-olefin/vinylidene aromatic monomer or hindered aliphatic vinylidene monomer interpolymers.
Elastomers are defined as materials which experience large reversible deformations under relatively low stress. Elastomers are typically characterized as having structural irregularities, non-polar structures, or flexible units in the polymer chain. Some examples of commercially available elastomers include natural rubber, ethylene/propylene (EPM) copolymers, ethylene/propylene/diene (EPDM) copolymers, styrene/butadiene copolymers, chlorinated polyethylene, and silicone rubber.
Thermoplastic elastomers are elastomers having thermoplastic properties. That is, thermoplastic elastomers may be molded or otherwise shaped and reprocessed at temperatures above their melting or softening point. One example of thermoplastic elastomers is styrene-butadiene-styrene (SBS) block copolymers. SBS block copolymers exhibit a two phase morphology consisting of glassy polystyrene domains connected by rubbery butadiene segments. At temperatures between the glass transition temperatures of the butadiene midblock and the styrene endblocks, that is, at temperatures from -90.degree. C. to 116.degree. C., the SBS copolymers act like a crosslinked elastomer.
European Patent Publication 416,815 discloses pseudorandom ethylene-styrene interpolymers. Uncrosslinked pseudorandom ethylene/styrene interpolymers exhibit a decreased modulus at temperatures above the melting or softening point of the interpolymer.
SBS copolymers and uncrosslinked ethylene-styrene pseudorandom interpolymers suffer the disadvantages of relatively low mechanical strength, susceptibility to ozone degradation (to the extent that they have sites of unsaturation in the polymer backbone), and utility in only applications where the temperature of the elastomer will not exceed the melting or softening point of the elastomer.
In contrast, thermoset elastomers are elastomers having thermoset properties. That is, thermoset elastomers irreversibly solidify or "set" when heated, generally due to an irreversible crosslinking reaction. Two examples of thermoset elastomers are crosslinked ethylene-propylene monomer rubber (EPM) and crosslinked ethylene-propylene-diene monomer rubber (EPDM). EPM materials are made by the copolymerization of ethylene and propylene. EPM materials are typically cured with peroxides to give rise to crosslinking, and thereby induce thermoset properties. EPDM materials are linear interpolymers of ethylene, propylene, and a nonconjugated diene such as 1,4-hexadiene, dicyclopentadiene, or ethylidene norbornene. EPDM materials are typically vulcanized with sulfur to induce thermoset properties, although they alternatively may be cured with peroxides. While EPM and EPDM materials are advantageous in that they have applicability in higher temperature applications, EPM and EPDM elastomers suffer the disadvantages of low green strength (at lower ethylene contents), of a higher susceptibility of the cured elastomer to attack by oils than characteristic of styrene butadiene rubbers, and of resistance of the cured elastomer to surface modification.
Elastomers suitable for use over a broad range of temperatures and which are also less susceptible to ozone degradation are desired. Thermoset elastomers which are prepared from elastomers having high green strength (which provides greater flexibility in their handling prior to curing) are particularly desired. Also desired, are thermoset elastomers which are resistant to oil, which are useful in fabricated parts which typically contact oil, such as automotive parts and gaskets. Also desired are thermoset elastomers which easily undergo surface modification, to promote surface adhesion of the elastomer and/or to provide ionic sites on the elastomer surface. Also desired is a process for preparing such thermoset elastomers.
Thermoplastic vulcanizates are crystalline polyolefinic matrices through which thermoset elastomers are generally uniformly distributed. Examples of thermoplastic vulcanizates include EPM and EPDM thermoset materials distributed in a crystalline polypropylene matrix. Such thermoplastic vulcanizates are disadvantageous, in that they are susceptible to oil degradation. Thermoplastic vulcanizates which are more resistant to oil are desired. Also desired is a process for preparing such thermoplastic vulcanizates.
Interpolymers prepared from .alpha.-olelfin/vinylidene aromatic monomer or hindered aliphatic vinylidene monomer have excellent properties; however, it would be desirable to have such polymers with improved properties.
It has been discovered that properties such as higher upper service temperature, improved melt processibility and self sticking tendencies of such interpolymers can be improved via crosslinking of the interpolymers.
The foams prepared from the cross-linked interpolyers are believed to have one or more of the following improvements: improved upper service temperature, lower density, improved elastic recovery properties, improved mechanical properties as compared to non-crosslinked interpolymer foams.